How to fix these hard Edges in Render?
Your issue is indeed caused by the model topology. One option is to use Subsurf modifier to smooth out the mesh. The problem with this approach is that you will loose lowpolyness of this object. Current 735 vs 70,370 verts after 3 level of subdivision (this level reduce artifacts completely).

As an alternative you can just smooth it with Sculpt Smooth brush.


Go to Sculpt Mode.
Select Smooth brush and set Strength to low value - I've used 0.1.
Smooth it!